1. Field of the Invention
The present invention relates to an apparatus and method for processing and displaying pictures, and particularly to a picture processing and displaying apparatus and method that process and display a photographed digital picture in a realtime fashion.
2. Description of the Related Art
Most digital cameras output photographed pictures on their liquid crystal display or the like in a realtime fashion, thus enabling the display to function as a viewfinder. Some digital cameras have a function of editing a source picture before it is output. Such editing functions include rotation of pictures and left-to-right reversal of pictures.
Realtime processing of a photographed picture starts with storing that picture in a memory for subsequent transform operations. The picture in the memory is edited and displayed on a monitor screen.
Since digital camera users require an increasingly high picture quality in recent years, the number of pixels constituting a digital picture keeps on growing. A digital camera may also be offered as an additional function of mobile phones and other electronic appliances, and higher pixel resolutions are still required in such applications. Actually many existing mobile phones are equipped with a built-in digital camera with resolutions of one million pixels or more. However, as the number of pixels of a picture increases, a larger memory capacity will be required to allow the user to edit that picture, thus resulting in increased product costs.
It would be possible, for example, to store a picture in a memory in compressed form and edit that compressed picture after decoding it in its entirety. Besides requiring a large capacity memory, this method has a large processing latency (i.e., a long delay time from photographing to displaying), which is quite unsuitable for the viewfinder applications mentioned above.
As can be seen from the above, decoding a picture for the purpose of subsequent editing essentially requires a memory capable of storing a decoded picture. It is therefore desirable to develop a function that can edit a picture while decompressing compressed data, but not storing every decoded image. In that case, it is necessary to start decompressing compressed data at any block or unit image constituting compressed data.
Compressed data contains image components of each block in coded form, and those coded components of a block may include information about differences between blocks. Take a coding method according to the Joint Photographic Coding Experts Group (JPEG), for example. This method encodes a picture on an individual block basis. The coded data includes direct current (DC) components coded in the form of block-to-block differences. Suppose now that it is required to decode a particular area of such differential-coded data. The required data, however, can only be obtained by decoding blocks sequentially from the very first block.
To address the above drawback, there is a technique that enables compressed data to be decoded from any block. Specifically, this technique defines a “starting block” beforehand so as to decode a compressed image file from an intermediate point. The term “starting block” refers to a block that will be located at the topmost row when an editing operation (e.g., rotation by 90 degrees) is done. The proposed technique decodes a compressed image file, while saving a file pointer and DC components (before differential coding) of the starting block. The proposed technique then decodes a picture from the starting block of the compressed data. Because its own DC component has already been stored, the starting block can be decoded without the need for calculating its difference from other blocks (see, for example, Japanese Patent Application Publication No. 2001-86318).
According to the technique disclosed in Patent Literature 1, however, it is necessary to decode compressed data to store the DC component of the starting block. This decoding leads to an increased processing latency, while relaxing memory capacity requirements. The technique of Patent Literature 1 is therefore unsuitable for realtime applications.